Scientists have yet another complication to get to grips with as they try to understand how the climate works and how it might change in the coming century.

Researchers have found a discrepancy in the way ocean water warms compared with the air just above it. It seems that, in the last 20 years, water temperatures around the globe have actually risen faster than air temperatures.

The anomaly may have led climatologists to overstate the amount by which they believe the Earth as a whole has warmed in the last two decades.

The research team that discovered the discrepancy says their study underlines once again the need for a comprehensive and consistent system of climate observation across the whole globe.

Scientific buoys

John Christy from the University of Alabama, US, and colleagues examined the accuracy of methods used to determine air and water temperatures over the Earth's oceans. Traditionally, this has included ships' crews measuring temperatures in the water hauled on to deck in insulated buckets or passing through engine intakes.

In building a picture of the global climate, the practice has been to use water temperatures taken in these ways as a proxy for sea-air temperatures (which are problematic to record directly on the deck of a ship).

But when Christy's team examined all available data, including 20 years of information from scientific buoys in the Pacific that record sea and air temperatures simultaneously, they found a discrepancy between the seawater and real air-temperature measurements. Seawater one metre below the surface had warmed faster than air three metres above the surface, as measured since 1979.

According to the team, based on their findings and the use of real air-data, the Earth has warmed during the last 20 years at the rate of 0.13 deg C per decade - "slightly less" than the 0.18 deg C per decade if seawater proxy data is used in calculations.

Large and small

The study, which appears in the journal Geophysical Research Letters, goes right to the heart of the debate about global warming and the human-induced climate changes many now think are occurring.

Sceptics who doubt man is having much of an influence on global temperatures have always attacked the quality of climate monitoring and highlighted the discrepancy between measurements taken at ground level and those recorded several kilometres up in the atmosphere by balloons and satellites. The former show a rapid rise in temperatures; the latter, covering a shorter time period, show little or no warming.

David Parker, from the UK's Met Office, and a co-researcher on the study, said there were intriguing similarities between the satellite-balloon-ground anomaly and the one identified between sea air and seawater.

"These sound as though they may be physically consistent phenomena," he told BBC News Online. "What is happening on the grand scale may be happening on the smaller scale as well."

Better models

The study is likely to improve the scientific community's confidence in the satellite data, which cover more than 95% of the globe, including remote ocean, desert and wilderness regions for which traditional climate data are either scarce or not available at all.

Parker sat on a recent US panel that examined the satellite-balloon issue and concluded that the temperature trends observed several kilometres up and at ground level were real. But the panel called for "an improved climate monitoring system to resolve uncertainties in data and provide policy-makers with the best available information".

Parker said: "What is required is a proper, global climate observing system so that you have reliable data everywhere - both satellites and based on the ground, and that if anyone changes their instruments, they have to run parallel experiments to ensure they haven't introduced a bias into the measurements."

Accurate data and comprehensive explanations of climate processes are essential if scientists are to build reliable models to predict future climate change.